Multistage filter



Oct. 1, 1935. L, w, BLAU 2,016,264

I MULTISTAGE FILTER I Filed June 14, 1934 1A E A? A265 Patented Oct. 1,1935 Ludwig W. Blau, Houston,

Tex., assignor to Standard Oil Development Company, a corporation ofDelaware Application June 14, 1934, Serial No. 730,565

10 Claims. (01. 178-44) This invention relates to improvements in amulti-stage filter.

It has been known how to combine electric networks so as to render themfrequency selective; thus it is possible to make low pass filters, highpass filters or intermediate pass filters. Such a filter is the subjectof applicants copending application U. S. Serial No. 659,822, entitledMethod and apparatus for eliminating low frequencies, filed May 6, 1933.

When filters are to be included in electrical recording systems wherehigh accuracy is required, it becomes necessary to damp out alloscillations originating in the filter itself. Thisis done successfullyin the filter which is the subject of the above mentioned patentapplication. However, the introduction of damping renders the cut-off ofthe filter less sharp so that more stages must be added to make up forit. This means that the filter becomes larger and more expensive toconstruct. The invention which is the subject of this applicationpermits of building a damped filter of a few stages with a sharp cut-offheretofore obtained only by using many stages.

The invention will be fully understood from the following descriptiontakenin connection with the accompanying drawing in which latter Fig. lis a diagrammatic representation of a preferred form of circuit;

Fig. 2 is a diagrammatic representation of a modified form of circuit,and

Fig. 3 is a diagrammatic representation'of an arrangement of apparatusfor use in reflection shooting.

Referring particularly to Fig. l of the drawing, reference numeral Idesignates an input impedance and numeral 2 designates an outputimpedance. An electric circuit connects the impedances. A plurality ofgroups of stages of parallel branches 4, 5 and 6 are provided in thecircuit. Each group or stage includes a branch A having an inductance 9,a condenser I0 and a resistance ll of a choke coil. Each group or stagealso includes a branch B having a resistance I3 and a condenser I4connected in series. A condenser I6 is provided in the circuit betweenthe resistance l and the branch A of the stage 4, and a condenser I! isprovided in the circuit betweenthe branch B of the stage 6 and theoutput resistance 2. A damping resistance l9 and a condenser 20 areconnected in series in the circuit betweenthe adjoining stages 4 and 5.A damping resistance 2| and a condenser 22 are disposed in the circuitbetween the adjoining stages 5 and 6.

The circuit operates as follows: Such values of the inductance 9 andcondenser Ill of the branch 7 l are selected that the combinationresonates 5 at, for example, five cycles. The combination of inductance9 and condenser Ill in series and condenser I4 in parallel are tuned toresonate at, for example, approximately 30 cycles. Then at a frequencyof five cycles the voltage across the 10 inductance 9 and condenser H1in series will be zero. In other Words, any frequency of five cyclesimpressed upon the input resistance I. will be practically eliminated bythe branch A. On

the other hand, at 30 cycles the inductance a 1'5 and condenser 10 inseries will resonate with the condenser M in parallel, and the voltagetransmitted across the combination of branches A and B therefore be veryhigh. This combinav tion of elements results in passing the waves of 2030 cycle frequency impressed upon the input resistance l practicallywithout attenuation through stage 4.

Referring to stage 5, the inductance 9 and condenser ID in series aretuned to resonate at, 25

for example, 10 cycles. The combination of inductance 9 and condenserIll in series of branch A togetherwith the inductance M in parallel ofbranch B is tuned to resonate at 30 cycles. Then any 10 cycle frequencyimpressed upon the input 36 resistor I will set up practically zerovoltage across the inductance 9 and condenser Ill of stage 5. In otherwords, any frequency of 10 cycles impressed upon the input resistance Iwill be practically eliminated in branch A of stage 5. At 30 35 branch Aof this stage may be tuned to resonate at, for example, 15 cycles,whilethe parallel combination comprising inductance 9 and condenser illin series of stage 6 due to an impressed frequency upon the resistor lof 15 cycles will be practically zero, so that any 15 cycle frequencywill be practically eliminated. On the other hand, since theparallelcombination tunes at 30 cycles any frequency of 30 cycles impressed uponthe input resistor I will set up a high voltage across the combinationand will therefore be 6 5" prises the parallel branches A and B as previously described.

Referring particularly to Fig. 3, .a preferred '7 arrangement ofapparatus for reflection shooting is shown in which reference numeral 28designates the coil of a magneto-electrical seismograph such as isdescribedin the copending application Serial No. 626,132 of Ludwig W.Blau et a1, entitled Seismograph, filed July 29, 1932. The coil 28 isconnected across the grid and" filament of a vacuum' tube" 29 which maybe a three electrode. or screen grid tube. The vacuum tube 29'isconnected to the input resistance I of the circuit previously describedin connection with Fig. 1. The output resistance 2 of the circuit isconnected to a vacuum tube amplifier 30' which in turn isconnected to anoscillograph 3! which records the seismograms. Preferably the filtercircuit is connected aheadof the amplifier 30 so that the objectionablelow! frequencies can be eliminated before being amplified.

In the circuits described, the resistances I3 in branch Bof each stageof the filter function to damp out oscillations set up in the parallelcombination formed by the two branchesA and B of each stage. Without theresistors I3 0b-' noxious oscillations originating in the respectivestages would appear in the form of voltages across the output resistorand would therefore be passed on. This would 'be detrimental to thequality of the seismogram since great 'care must always be taken torecord the filtered earth motion as faithfully as possible and' to haveno ex-' traneous frequencies on the seismogram originating in the filteror any other parts of the seismograph apparatus. These extraneousfrequencies would lead to misinterpretations of the results.

The resistors 19 and 2| are likewise damping resistors designed to dampout oscillations of the following circuits. The resistor l9 damps outoscillations in the circuit consisting of inductance 9 and condenser ID,of branch A of stage 4, condenser l0 and inductance 9 of branch A ofstage B, and condenser 20 which lies between these stages. The resistor19 also damps out oscillations in the circuit which comprises inductance9 and condenser I E! of branch A of stage 5, the condenser I4 of branchB of stage 4 and the condenser 20. The resistor l9 also damps outoscillations in the circuit comprising inductance 9 and condenser IQ ofbranch A of stage 6, condenser M of branch Bof stage 5 and condenser 20.The damping resistor 2I damps out oscillations in the circuit comprisinginductance 9 and condenser ll] of branch A of stage 5, condenser H! ofbranch A'of stagefi and condenser 22. The resistance 2| damps out alsooscillations in the circuit comprising inductance 9 and condenser ID ofbranch A of stage 5, the condenser I4 of branch B of stage 6 andcondenser 22. tions in the circuit comprising condenser [4 of Theresistor 2| damps out oscilla- I branch B of stage 5, inductance 9 andcondenser ID of branch A of stage 6 and condenser 22. The input resistorI damps out oscillations originating in the circuit comprising thecondensers i6 and In in the inductance 9. The output resistor 2 dampsout oscillations originating in the circuit comprising the condenser Hand the inductance l3 and condenser M of branch B of stage 6.

By reason of the parallel combination 'of branches as described whichmay be arranged to tune at any predetermined frequency, which in thecase described has been assumed to be 30 cycles, this filter will passall or practically all energy impressed upon the input with frequencyequal to or greater than 30 cycles. The filter here described thereforehas a much sharper cut-oif with a smaller number of stages than has.heretofore been possible to attain. This filter renders possible thepractically complete elimination of all frequencies below any certainpredetermined cut-off frequency, while passing all energy orpracticallyall energy of frequency higherth'an the cut-off frequencywhich is impressed upon the input resistance 1 or the transformer 24. i1

Various changes may be made within the scope of the appended claims inwhich it is desired to claim all novelty inherent in the invention asbroadly as the prior art permits.

Iclaim:

1.. A filter, 'compri'sing'an electric circuit having a plurality ofstages, each stage including a branch having a condenser'and aninductance connected in series and tuned to a given frequency, thecorresponding branch of each stage being tuned to different frequencieswhereby these frequencies are eliminated,'each stage in cluding a branchhaving a condenser, the two branches of each stage being tuned toresonate together at a given frequency whereby this frequency is passedthrough the filter.

2. A filter, comprising an electric circuit having a plurality ofstages, each stage including a branch having a condenser and aninductance connected in series and tuned to resonate at a' givenfrequency, the. corresponding branch of each stage being tuned, todifferent frequencies whereby these frequencies are eliminated, eachstage including a branch having a condenser, D

the two' branches of each stage being tuned to resonate together at thesame frequency outside of the range of, the first mentioned frequencieswhereby this frequency is passed through the filter.

3. A filter, comprising an electric circuit. having a plurality ofstages, each stage including a branch having a condenser and aninductance connected inseries and tuned to resonate at a givenfrequency, the corresponding branch of each stage being tuned todifferent frequencies whereby these frequencies are eliminated, eachstage including a branch having a condenser, the two branches of eachstage being tuned to resonate together at the same frequency outside ofthe range of the first mentioned frequencies whereby this frequency ispassed through the circuit and a damping resistance in the circuitbetween adjoining stages for damping out the natural frequencies of thefilter. I

4. A filter, comprising an electric circuit having 70 each stage beingtuned to different frequencies whereby these frequencies are eliminated,each stage including a branch having a condenser and a resistanceconnected in series, the two branches of each stage being tuned toresonate together at a given frequency outside of the range of the firstmentioned frequencies whereby this frequency is passed by the filter,and a damping resistance in the circuit between adjoining stages fordamping out the natural frequencies of the filter.

5. A filter, comprising an electric circuit having a plurality ofstages, each stage including a branch having a condenser and aninductance and a resistance connected in series and tuned to a givenfrequency, the corresponding branch of each stage being tuned todifferent frequencies, each stage including a branch having a condenserand a resistance connected in series, the two branches of each stagebeing tuned to resonate together at a given frequency, and a dampingresistance for damping out the natural frequencies of the filter and acondenser in the circuit between adjoining stages.

6. A filter, comprising an electric circuit having a plurality ofstages, each stage including a first shunt arm having a condenser, aninductance and a resistance connected in series, and a second shunt armhaving a condenser connected in parallel with the first shunt arm, thefirst shunt arm of each stage being tuned to different low frequencieswhereby these low frequencies are eliminated, the second shunt arm ofeach stage being tuned with respect to the first shunt arm of that stageto resonate at higher frequencies whereby the higher frequencies arepassed.

'7. A filter for eliminating low frequency waves, comprising theinductance coil of a seismograph connected across the grid and filamentof a vacuum tube, an input impedance connected to the plate resistor ofthe vacuum tube, an output impedance, an electric circuit connecting theimpedances and including a plurality of stages of parallel shunt arms,each stage including a first shunt arm having a condenser, an inductanceand a resistance connected in series and a second shunt arm having acondenser connected in parallel with the first shunt arm, the firstshunt arm of each stage being tuned to different low frequencies wherebythese low frequencies are eliminated, the second shunt arm of each stagebeing tuned with respect to the first shunt arm of that stage toresonate at higher frequencies whereby the higher frequencies arepassed, a vacuum tube amplifier connected to the output impedance, andan oscillograph connected to the amplifier.

8. A filter for eliminating low frequency waves, comprising theinductance coil of a seismograph connected across the grid and filamentof a vacuum tube, an input impedance connected to the plate resistor ofthe vacuum tube, an output impedance, an electric circuit connecting theimpedances and including a plurality of stages, each stage including abranch having a condenser and an inductance and a resistance connectedin series and tuned to a given frequency, the corresponding branch ofeach stage being tuned to different frequencies, each stage including abranch having a condenser and a resistance connected in series, the twobranches of each stage being tuned to resonate together at a givenfrequency, a damping resistance for damping out the natural frequenciesof the filter and a con-' denser in the circuit between adjoiningstages, a

vacuum tube amplifier connected to the output impedance and anoscillograph connected to the amplifier.

9. A filter for eliminating low frequency waves,

comprising the inductance coil of a seismograph connected across thegrid and filament of a vacuum tube, an input impedance connected to theplate resistor of the vacuum tube, an output impedance, an electriccircuit connecting the impedances and including a plurality of stages ofparallel shunt arms, each stage including a first shunt arm having acondenser an inductance and a resistance connected in series and asecond shunt arm having a condenser and a resistance in series, thelatter shunt arm being connected in parallel with the first shunt arm,the first shunt arm of each stage being tuned to different lowfrequencies whereby these low frequencies are eliminated, the secondshunt arm of each stage being tuned respectively to the first shunt armof that stage to resonate at higher frequencies whereby the higherfrequencies are passed, a condenser connected between the inputimpedance and the first shunt arm, a condenser connected between thelast shunt arm and the output impedance, a vacuum tube amplifierconnected to the output impedance and an oscillograph connected to theamplifier.

10. A filter for eliminating low frequency waves,

comprising the inductance coil of a seismograph connected across thegrid'and filament of a vacuum tube, an input impedance connected to theplate resistor of the vacuum tube, an output impedance, an electriccircuit connecting the impedances andincluding a plurality of stages ofparallel shunt arms, each stage including a first shunt arm having acondenser and inductanceand a resistance connected in series and asecond a shunt arm having a condenser and resistance connected inseries, the latter shunt arm being connected in parallel with the firstshunt arm, the first shunt arm of each stage being tuned to differentlow frequencies whereby these low fre quencies are eliminated, thesecond shunt arm of each stage being tuned with respect to the firstshunt arm of that stage to resonate at higher frequencies whereby thehigher frequencies are 7 passed, a condenser and resistance in seriesconnected between the input impedance and the first shunt arm, acondenser and resistance in series 7 connected between the last shuntarm and the output impedance, a vacuum tube amplifier connected to theoutput impedancaand an oscillograph connected to the amplifier.

